In the manufacture of rubber such as for example butyl rubber, the butyl rubber product is collected in the form of bales on a packaging line where polyethylene based film is wrapped around the bale, the ends of the polyethylene film are welded together to enclose the bale and form a wrap or bag, and whereafter the bagged bales are boxed, sometimes in five layers on top of each other ready for shipment. The bales to be packaged sometimes are still warm from the manufacturing process and the bales may also be placed in the box in a compressed condition to save space.
On arrival at the manufacturing location the bales are extracted from the box and the bagged bales are then processed e.g. for tyre manufacture. The polyethylene based film thus becomes incorporated into the butyl mix. As a consequence the polyethylene based film used for packaging has to be compatible and meet specified criteria so as to conform to butyl rubber quality control requirements.
The quality control requirements may include the Vicat softening point and speaking generally this should be low i.e. below approximately 82.degree. C. taking into account margins for statistical process control. At the same time the bales are pushed out in a heated condition (up to 70.degree. C.) and output may have to be controlled so as to avoid bale wrapping difficulties. The bales are then put together in the box and should not adhere to one another so that the bales can be fed individually into processing equipment by the customer, the tyre manufacturer after shipment. Thus there is a requirement that the PE film should not stick together and simultaneously that such film should be soft. Compatible bale wrap films have to possess precisely defined physical properties in terms of response to bale wrapping temperatures, sticking and softness.
Known polyethylene based film used for butyl rubber wrapping is a copolymer of ethylene and vinyl acetate containing from 6 to 12% VA. The vinyl acetate derived polymer units reduce the crystallinity and lower the Vicat. However, the amount of vinyl acetate cannot be made too high to reduce the Vicat as this will cause sticking together of the bales in the box. Such known polyethylene based films also incorporate an anti-blocking additive in the form of silica in combination with a slip agent in the form of erucamide to enable higher levels of vinyl acetate without increasing stickiness. Even using the anti-blocking additive, difficulties may be encountered in formulating a film which does not stick together and which at the same time has a low Vicat.
JP-9019068 discloses a mixture of EVA and wax for treating a cable end which EVA has a melt index of greater than 300. This suitable for forming a hot melt extrusion but too high for film formation.
U.S. Pat. No. 4,235,657 discloses a web with two polymer film layers. A separable layer is coated on which contains EVA and an emulsifiable polyethylene wax. The EVA has a high melt index consistent with coating application. There is no requirement for a film forming, heat sealable, non-blocking composition.
Wax has also been used in hot melt adhesive formulations containing as other principal components EVA and a tackifier. The formulations have processing characteristics suitable for hot melt coating etc and are not extrudable into films. The waxes used may be crystalline waxes which are used in this application for rheology control and undesirably reduce adhesiveness.
GB-1516420 uses a low grade polyolefin (having an Mn of 2000 in Example 8) together with a polyolefin resin (such as EVA) and a tackifier to make a tacky film. A grease wax is preferred. The nature of the low grade polyolefin materials is not described. Penetration values are not indicated.
EP-100843 discloses a blend a narrow molecular weight distribution high mol weight polyethylene with a relatively low molecular weight ethylene polymer to give a bimodal distribution. The low molecular weight material has an MI of from 45-300 and an Mn of .about.5000, also with a narrow molecular weight distribution (Mw/Mn of from 2 to 4). Such low molecular weight material has a molecular weight which is too high to function in a waxlike manner.
It is the object of the invention to provide an extrudable film forming composition, either as a single layer or as part of a multilayer structure, which does not stick easily to other such films yet which has an acceptable VICAT to facilitate admixture and co-compounding of the film in rubber processing.
The invention has also as its aim to provide a polymer composition which provides a greater processing latitude and provides non-stick characteristics at more elevated temperatures, lower Vicat softening points, and/or higher percentages of vinyl acetate in the polyethylene based polymer for a given Vicat softening point or sticking tendency.